`whileDescendantOf` config for fork point override isn't just redundant, it's probably erroneous

[PawelLipski]

Repro:

1. 2 unique commits on a branch Y
2. delete its base branch X
3. override fork-point for Y to Y~2
4. git machete squash while on Y
5. fork point override to Y~2 no longer applies, due to whileDescendantOf config, even though that's still logically the fork point (and is pointed by to)

Time to deprecate the setting: newer version of git-machete doesn't need to set this config at all. Backward compatibility is going to be retained (as config set by an earlier version will be accepted by a newer version... just not the other way round, but we don't care about forward compatibility).